Biochemistry

Tic Toc

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Science  09 Aug 2002:
Vol. 297, Issue 5583, pp. 901
DOI: 10.1126/science.297.5583.901c

Chloroplasts (and mitochondria) are intracellular organelles defined by an inner and an outer membrane. Many of their constituent proteins are synthesized in the cytosol and thus need to traverse one or both membranes to reach their destinations. In an improved in vitro system, Hinnah et al. have measured the electrophysiological properties of Toc75, which forms the core of the peptide-translocating channel across the outer chloroplast membrane. Analysis of the currents as a function of voltage, ionic composition, and transit peptide sequence leads them to describe the channel as a constricted central region 1.4 nm in diameter, with a vestibule that is almost twice as wide. After crossing the outer membrane, imported proteins encounter the Tic complex; Heins et al. provide electrophysiological data indicating that Tic110 is the main component of this translocating channel, with a central filter of 1.5 nm. — GJC

Biophys. J.83, 899 (2002); EMBO J.21, 2616 (2002).

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